tl;dr: Herdr is a terminal multiplexer purpose-built for coding agents, letting you run multiple agents (Claude Code, Codex, Cursor, etc.) in persistent PTY sessions on any machine and attach from any terminal, including over SSH from a phone. Unlike tmux/Zellij, it understands agent state (blocked/working/done) and exposes a CLI and JSON socket API so agents can orchestrate panes themselves. It's a single binary with no Electron, account, or telemetry, available on Linux/macOS with a Windows preview.